A confocal scanning laser microscope for quantitative ratiometric 3D measurements of [Ca2+] and Ca2+ diffusions in living cells stained with Fura-2

Proposal of a New Method for Measuring Förster Resonance Energy Transfer (FRET) Rapidly, Quantitatively and Non-Destructively

The process of radiationless energy transfer from a chromophore in an excited electronic state (the “donor”) to another chromophore (an “acceptor”), in which the energy released by the donor effects an electronic transition, is known as “Förster Resonance Energy Transfer” (FRET). The rate of energy transfer is dependent on the sixth power of the distance between donor and acceptor. Determining FRET efficiencies is tantamount to measuring distances between molecules. A new method is proposed for determining FRET efficiencies rapidly, quantitatively, and non-destructively on ensembles containing donor acceptor pairs: at wavelengths suitable for mutually exclusive excitations of donors and acceptors, two laser beams are intensity-modulated in rectangular patterns at duty cycle ½ and frequencies f₁ and f₂ by electro-optic modulators. In an ensemble exposed to these laser beams, the donor excitation is modulated at f₁, and the acceptor excitation, and therefore the degree of saturation of the excited electronic state of the acceptors, is modulated at f₂. Since the ensemble contains donor acceptor pairs engaged in FRET, the released donor fluorescence is modulated not only at f₁ but also at the beat frequency Δf: = |f₁ − f₂|. The depth of the latter modulation, detectable via a lock-in amplifier, quantitatively indicates the FRET efficiency.

Involvement of intracellular free Ca2+ in enhanced release of herpes simplex virus by hydrogen peroxide

Background

It was reported that elevation of the intracellular concentration of free Ca²⁺ ([Ca²⁺]i) by a calcium ionophore increased the release of herpes simplex virus type 1 (HSV-1). Freely diffusible hydrogen peroxide (H₂O₂) is implied to alter Ca²⁺ homeostasis, which further enhances abnormal cellular activity, causing changes in signal transduction, and cellular dysfunction. Whether H₂O₂ could affect [Ca²⁺]i in HSV-1-infected cells had not been investigated.

Results

H₂O₂ treatment increased the amount of cell-free virus and decreased the proportion of viable cells. After the treatment, an elevation in [Ca²⁺]i was observed and the increase in [Ca²⁺]i was suppressed when intracellular and cytosolic Ca²⁺ were buffered by Ca²⁺ chelators. In the presence of Ca²⁺ chelators, H₂O₂-mediated increases of cell-free virus and cell death were also diminished. Electron microscopic analysis revealed enlarged cell junctions and a focal disintegration of the plasma membrane in H₂O₂-treated cells.

Conclusion

These results indicate that H₂O₂ can elevate [Ca²⁺]i and induces non-apoptotic cell death with membrane lesions, which is responsible for the increased release of HSV-1 from epithelial cells.

Confocal laser scanning microscopy of calcium dynamics in living cells

Confocal laser scanning microscopy (CLSM) is widely used to monitor intracellular calcium levels in living cells loaded with calcium-sensitive fluorophores. This review examines the basic advantages and limitations of CLSM in in vivo imaging analyses of calcium dynamics. The benefits of utilizing ratioed images and dextran-conjugated fluorophores are addressed, and practical aspects of handling confocal datasets are outlined. After considering some relatively new microscopical methods that can be used in conjunction with conventional CLSM, possible future applications of confocal techniques in analyses of intracellular calcium dynamics are discussed.

A study of the precision of confocal, ratiometric, Fura-2-based [Ca2+] measurements

The precision with which quantitative confocal ratiometric Fura-2-based calcium measurements can be performed has been investigated. A standard confocal scanning laser microscope system has been modified so as to enable excitation with the 351 nm and 364 nm lines of the UV argon laser and simultaneous separation of the fluorescence emanating from the two different excitation wavelengths. Experiments have been performed on living cells and mainly on Fura-2 containing calibrated Ca2+ buffer solutions in order to study the variation of the precision with various experimental parameters in a controlled manner. Furthermore, expressions relating the precision of the calcium determination to various measured parameters are derived. These expressions have been used in conjunction with published excitation intensity spectra for Fura-2 in order to verify the experimental results and to analyse the variation of parameters that could not be experimentally tested. The results show that a photon number of the order of 10,000 per pixel, at maximum Ca2+ concentration and 351 nm excitation, is required in order to reduce the relative error at all concentrations to less than 30%. The commercially available laser wavelength combinations, 325 nm/380 nm and 334 nm/380 nm, were found to be the best as concerns the precision of the method.